In honor of National Running Day, here are some interesting tidbits about running and racing.

Runners are getting adventurous…and dirtyLast year, more people completed obstacle races and mud runs than marathons and half-marathons. These non-traditional races had an estimated 4 million finishers in 2013. (But the 5K is still the most popular race distance, with more than 6.2 million finishers in 2012, says Running USA.)

Your boobs move way more than you thinkResearchers from the University of Portsmouth in England have found that breasts don’t just bounce up and down during running; they also move side to side and in and out, creating a complicated figure-8 pattern. This so-called breast displacement could permanently damage the Coopers ligaments and lead to premature sagging. That’s why it’s super important to have supportive sports bras that minimize bouncing as much as possible, but are comfortable enough that you don’t hate to wear them.

You’re fastest in your 20sOver the marathon distance, at least. Spanish researchers looked at the ages and finishing times of runners in the 2010 and 2011 New York City Marathon and found that men run their fastest marathon at age 27; women at 29. Times were 4% slower for every year younger than these ages, and 2% slower for each year thereafter. Meaning an 18-year-old marathoner runs about as fast as a 60-year-old finisher.

…but people run long distances well into their 90sThe oldest person to run a marathon is Fauja Singh, who at 101 completed the 2012 London Marathon in 7:49:21. The oldest female marathon finisher on record is Gladys Burrill, who at age 92 ran the 2010 Honolulu Marathon in 9:53:16, according to Runner’s World.

Running burns lots of caloriesThe old saw that running torches about 100 calories per mile is a good benchmark, but calorie burn really depends on weight, and your pace and fitness level also come into play. A good formula is to multiply your weight in pounds by 0.63, then multiply by the number of miles. The number you get is the amount of calories you burned over and above the basal metabolic rate (calories you burn just sitting around). If a 150-pound woman runs 6 miles, she’ll burn about 567 calories. But as you get fitter, running the same number of miles won’t burn as many calories. Then it’s time to go faster—or longer.

Running can be a job requirementThe Army’s Basic Training Physical Fitness Test has three parts: push-ups, sit-ups, and a two mile run. Check out the minimum times to pass the running portion, below. You have to run even faster as a soldier to meet the requirements for the twice-annual Army Fitness Test, or to pass Advanced Infantry Training.

Women rule the roadsWomen made up 56% of all 15.5 million race finishers in 2012, according to Running USA. The half-marathon distance has the biggest female presence with a whopping 61/39 gender split. In fact, the marathon is the only event where men have women beat in terms of sign-ups.